When Saul was instructed to attack the Amalekites, the LORD made it clear that there was to be no plunder (1 Samuel 15:1–3). It is not clear why David is allowed to keep the plunder. Perhaps it was due to the fact that he was on a rescue and recovery mission whereas Saul was instructed to exercise a scorched-earth policy.1
20 David also captured all the flocks and herds, and the people drove the livestock before him, and said, “This is David’s spoil.”